We had an offer from UU to have a meter fitted for free, for a 13 month trial period. They sent us an estimate of what they thought our bills would be (don't know how they knew our usuage!) which showed that we'd save 25% off our bills. They also included a ready reckoner for us to do our own calculation. We worked this out on the generous side, and it showed that we'd be paying only 50% of the non metered cost. So we went with it, and after a year our bills were only 30% of the previous cost. A massive saving so we're well happy. We don't have a tribe of nappy wearing kids, but we don't skimp on water, use washing machine & dish washer when we need to and we have a fish pond that's topped up once a week. We do use a shower rather than a bath, but wash and hose our cars when they are dirty.
See if your water people will let you have a free trial like we did.